Ceramic-to-metal seals are required in certain processes which operate at high temperatures and which may operate in chemically active environments. Such seals are required in high temperature ceramic heat exchangers, fuel cells, gas sensors, hot gas filters, and ceramic membrane reactors producing oxygen or synthesis gas. A common problem in the design and operation of such seals is that ceramics and metals usually have different coefficients of thermal expansion, which can cause seal failures due to mechanical stresses in the seals during heating and cooling.
One type of seal for joining ceramics and metals utilizes selected mixtures or combinations of ceramics, glasses, or metal brazing compositions to form a graduated seal fused to the ceramic and metal elements to be joined. One method for joining ceramic materials having dissimilar coefficients of thermal expansion is the graded glass seal. This type of seal is used to join a low expansion glass such as Pyrex.TM. or fused silica to a relatively higher expansion ceramic such as alumina or mullite. The seal is formed by forming several layers of glasses with coefficients of thermal expansion intermediate the two end members such that the coefficient of thermal expansion of the seal forms a gradual transition from one end member to the other. These types of seals, while gas tight, are limited to relatively low operating temperatures (.ltoreq..about.600.degree. C.) and pressures near ambient.
An alternative approach has been the use of soft metal brazes such as alloys of silver and gold. This type of seal can accommodate small differences in coefficients of thermal expansion; however, it is limited in operating temperature and pressure by the melting point of the alloys and the high temperature deformation resistance of the alloys. Thermal cycling of this type of seal can result in cracking of the ceramic if the coefficient of thermal expansion of the ceramic is significantly different than that of the alloy.
Seals of the types described above can be used in reactors utilizing mixed conductor metal oxide ceramic membranes which conduct oxygen ions. Representative or illustrative examples of this application for fused seals are given in U.S. Pat. Nos. 5,599,383, 5,561,373, 5,712,220, and 5,725,218.
Alternatively, metal and ceramic elements can be sealed by mechanical sealing devices which contact, but are not fused to, the metal and ceramic materials to be sealed. Such sealing devices may allow some movement between the metal and ceramic members, thereby relieving stresses caused by different degrees of expansion during heating and cooling.
U.S. Pat. No. 5,358,262 describes a multi-layer seal element for use between metal and ceramic components at high temperatures. The element is composite of elongated ceramic fibers, braided metal mesh, and braided ceramic fiber, and the composite element can be formed into an O-ring for use in a flanged seal.
A seal for a ceramic gas sensor element is disclosed in U.S. Pat. No. 5,795,454 in which the sensor is held in a longitudinal metal bore by a stack of compressed sintered ceramic sealing bodies. A similar seal is disclosed in German Patent Publication DE 195 32 090 A1.
U.S. Pat. No. 5,401,406 describes a seal device for sealing a high temperature ceramic filter element into a metal housing wherein a ceramic fiber gasket material is compressed between metallic and ceramic elements which have different coefficients of thermal expansion. Another type of high temperature ceramic filter is disclosed in U.S. Pat. No. 4,735,635 in which tubular ceramic filter elements, each having an enlarged shoulder on the open end, are inserted into a metal tube sheet having holes smaller than the tube shoulders. High temperature gasket material is placed between each tube shoulder and the tube sheet, and the gasket is compressed in place by exerting compressive force on the ends of the ceramic tubes.
A high temperature ceramic-to-metal seal for a ceramic heat exchanger is disclosed in an article entitled "Development of a High-Temperature Ceramic to Metal Seal" by S. B. M. Beck et al in Proc Instn Mech Engrs Vol 211 Part E, pp. 109-114. The seal utilizes a stuffing box with a woven alumina rope packing material which is compressed in place by a screw attachment which urges the packing material against the metal and ceramic parts to be sealed.
Ceramic-to-metal seals for solid electrolyte ionic conductor reactors are described generically by U.S. Pat. Nos. 5,820,654 and 5,820,655 in which seals are provided at less than 300.degree. C. by welding or brazing between ceramic and metal members. It is stated that O-rings, bellows, or other mechanical means can be used. It is disclosed that ceramic tubes can be sealed into tube sheets by sliding O-ring seals of unspecified material and design.
Metal and ceramic components can be sealed by means of flexible metal bellows which are welded or brazed in place as disclosed in articles entitled "Catalytic Inorganic Membrane Reactors: Present Experience and Future Opportunities" by G. Saracco et al in Catal. Rev., Sci. Eng., 36(2), 305-384, at pp. 366-368 and "Development of a High Temperature Resistant Module for Ceramic Membranes" by F. M. Velterop et al in Key Engineering Materials, Vols. 61 and 62 (1991), pp. 391-394.
The design and operation of high temperature mixed conductor membrane reactor systems for the production of oxygen, synthesis gas, and other hydrocarbon products will utilize tubular geometry within the reactor modules and for piping connections to the reactor modules for feed and product gas flow. Ceramic-to-metal seals are required in these reactor systems to segregate feed and product gases at elevated process temperatures in the range of 500.degree. C. to 1000.degree. C. Such seals must be able to cycle between ambient temperature and operating temperature while segregating gases with elevated pressure differentials across the seals. The invention disclosed below and defined by the claims which follow provides compliant mechanical seals for such high-temperature applications, in particular for use in the operation of ceramic membrane reactor systems.